High resolution light microscopy is limited not only by the diffraction properties of light but also the optical inhomogeneities of the sample. Adaptive optics and deconvolution techniques can be used to correct sample-induced aberrations if the aberrations due to the refractive index variations are adequately measured. To be most effective, however, both correction strategies require quantitative knowledge of the 3-D position-dependent refractive index of the sample.
The refractive index reveals a unique aspect of cellular structure, and is important in the measurement of cell and tissue light scattering, laser trapping of single cells, flow cytometry, total internal reflection microscopy, and generally involving the interaction of light with cells and tissues.
Prior methods for cellular index measurement typically provide the average refractive index of a cell, ignoring spatial variations due to sub-cellular structure. Moreover, they require immersion of cells in liquids of various refractive indices and subsequent observation with phase contrast microscopy. This procedure is cumbersome and is limited by the tendency of some cells to be altered by the immersion liquids, which are typically not physiologically controlled. More recently, accurate measurements of average index have been performed using quantitative phase microscopy techniques, but measurement of three-dimensional spatial variation has not been possible.